The invention relates to a microwave oscillator wherein a first oscillator diode is injection-locked with electromagnetic energy from a second oscillator diode. Such an arrangement may be used where the first diode has a higher output power at the desired frequency than the second diode but has, for example, poorer frequency stability and/or more noise. The first diode may for example be an IMPATT diode and the second diode may for example be a Gunn diode. The invention is particularly suitable for providing an oscillator with an output frequency in the mm-wave range, for example 94 GHz, in which it is not otherwise readily possible to generate high-quality signals with solid-state devices at relatively high power levels.
The injection-locking of microwave oscillators is well known. (For an example of an injection-locked oscillator for the mm-wave range, see "Generating power at mm-wave frequencies" by N. B. Kramer and R. A. Johnson, Microwaves & RF, May 1984, pages 243-249, particularly FIG. 14 and the related text on pages 245-246.) Where, as is usually the case, the injected locking signal is of the same frequency as the final output signal from the oscillator, it is necessary to provide a circulator or isolator between the higher-output diode and the source of the locking signal in order to ensure that the former does not lock the latter. This adds significantly to the cost of the oscillator and also increases its size.
The paper "Subharmonically Injected Phase-locked IMPATT-Oscillator Experiments" by C. H. Chien and G. C. Dalman, Electronics Letters, Vol. 6, No. 8 (Apr. 16, 1970), pages 240-241, discloses an oscillator comprising an IMPATT diode which is supplied with a subharmonic locking signal via a coaxial bias circuit including a low-pass filter adjacent the oscillator. The IMPATT diode oscillates at about 8.5 GHz, and the locking signal may be half, 1/6th or 1/9th of that frequency. The low-pass filter inhibits the propagation of microwave energy at the frequency of the IMPATT diode between that diode and the bias circuit (in each direction); an isolator and a further low-pass filter are additionally used between the first-mentioned filter and the locking source.
UK Patent Application No. GB 2 133 649 A discloses an injection-locked oscillator wherein an oscillator diode having a fundamental frequency below and a second harmonic frequency above the cut-off frequency of a waveguide in which the diode is disposed is injection-locked (via a resonant-cap coupling structure of the oscillator and a probe extending into the waveguide adjacent the cap) with a locking signal whose frequency is approximately equal to the free-running fundamental frequency of the diode.